Research on single-wall carbon nanotubes (SWNTs) was put forth by Iijima (MRS Bull 1994 19:43-49). The structure of a SWNT consists of an enrolled graphene that forms a seamless cylinder. Several potential applications of SWNTs require large amounts of material. For these to be practical, a low-cost, scalable continuous manufacturing process is required.
SWNTs can be produced by the methods similar to the ones used in multi-wall nanotubes (MWNT) synthesis, but in much smaller quantities and with lower yields.
Traditionally, carbon arc synthesis (Journet. et al. 1997 Nature 756) or pulsed laser vaporization of carbon (Thess et al. Science 1996 273:483-487) in the presence of a metal catalyst (which is always required for SWNT synthesis) has been used. More recently, chemical vapor deposition (CVD) of benzene with a Fe catalyst was used to produce small amounts of SWNTs on substrates. Ni, Co and Fe work well as catalysts, but catalysts containing more than one element (e.g., Co—Pt or Ni—Y) increase the yield of SWNTs (Yumura, M. Synthesis and Purification of Multi-Walled and Single-Walled Carbon Nanotubes. in The Science and Technology of Carbon Nanotubes (eds. Tanaka, K., Yamabe, T. & Fukui, K.) 2-13 (Elsevier, Amsterdam, 1999)). Homogeneous catalytic decomposition of CO or HiPco (high pressure carbon monoxide) process was introduced in attempt to provide a continuous production method with a higher yield (Hafner et al. Chem. Phys. Lett. 1998 296). However, all of these techniques suffer from very low yields and lack of control over the tube size and chirality (Harris, P. J. F. Carbon Nanotubes and Related Structures (Cambridge University Press, Cambridge, 1999, 20-55). Large amounts of the catalyst trapped in the material and the presence of non-tubular carbon create additional difficulties, thus requiring the purification of nanotubes prior to use. Furthermore, the structure of individual tubes varies widely. Zigzag, armchair and chiral forms of various diameters coexist in the material.
Thus, there is a need for techniques for producing SWNTs in the amount and quality required for their industrial use.
Carbide-derived carbon (CDC) is produced in accordance with methods described by Gogotsi et al. (Nature 2001 411:283-287; Nature 1994 367:628-630; and J. Mater. Chem. 1997 7:1841-1848). In this method metals are extracted from carbides using reactive gases at elevated temperatures. For example, the formation of carbide-derived carbon (CDC) resulting from the extraction of silicon from SiC at 300-800° C. by supercritical water has been disclosed (Gogotsi, Y. G. and Yoshimura, M. Nature 1994 367:628-630). Extraction of metals from carbides by halogens (Cl2) or their compounds (HCl) has also been shown to lead to the formation of free carbon (Gogotsi et al. J. Mater. Chem. 1997 7:1841-1848). This method can be used to obtain carbon from SiC and other carbides that form volatile halides (SiCl4 is a typical example). SiCl4 is more thermodynamically stable than CCl4 at elevated temperatures. Thus, chlorine reacts selectively with the Si at SiC surfaces leaving carbon behind:SiC+2Cl2=SiCl4+C  (1)SiC+⅔Cl2=SiCl3+C  (2)Similar carbon formation occurs upon chlorination of other carbides or high-temperature dissociation of SiC (Motzfeldt, K. and Steinmo, M. Transport Processes in the Thermal Decomposition of Silicon Carbide. in Proceedings of The Ninth International Conference on High Temperature Materials Chemistry (ed. Spear, K. E.) 523-528 (The Electrochemical Society, Inc., Pennington, N.J., USA, 1997)). This is a versatile technology because a variety of carbon structures can be obtained (Gogotsi, Y. Nanostructured Carbon Coatings. in Proc. NATO ARW on Nanostructured Films and Coatings (eds. Chow, G.-M., Ovid'ko, I. A. & Tsakalakos, T.) 25-40 (Kluwer, Dordrecht, 1999). Not only simple shapes, but also fibers, powders and components with complex shapes and surface morphologies can be coated with carbon, and bulk carbon materials or powders can be produced by a reaction through the whole thickness of the powder or a monolithic component. This technology allows for control of the coating growth on the atomic level, monolayer by monolayer, with high accuracy and controlled structures.
The structure and morphology of CDC depends on the temperature and composition of the chlorinating gas mixture. Carbon films have been produced on β-SiC powders (Gogotsi et al. J. Mater. Chem. 1997 7:1841-1848), as well as on SiC based fibers (Gogotsi, Y. G. Formation of Carbon Coatings on Carbide Fibers and Particles by Disproportionation Reactions. in NATO ARW: Advanced Multilayered and Fiber-Reinforced Composites (ed. Haddad, Y. M.) 217-230 (Kluwer, Dordrecht, 1997)), monolithic CVD, and sintered ceramics (Gogotsi et al. Nanostructured Carbon Coatings on Silicon Carbide: Experimental and Theoretical Study. in Proc. NATO ASI on Functional Gradient Materials and Surface Layers Prepared by Fine Particles Technology (eds. Baraton, M. I. & Uvarova, I. V.) (Kluwer, Dordrecht, 2000)) exposed to Ar—H2—Cl2 gas mixtures at atmospheric pressure and temperatures between 600° C. and 1000° C. Carbon films with a thickness up to 200 μm have been formed as well on the surfaces of commercially available monolithic SiC specimens by high temperature chlorination at atmospheric pressure in Ar—Cl2 and Ar—H2—Cl2 gas mixtures (Ersoy et al. STLE Tribology Transactions 2000 43:809-815). The complete transformation of carbide powders to carbon has also been demonstrated (Gogotsi et al. J. Mater. Chem. 1997 7:1841-1848; Fedorov, N. F. and Samonin, V. V. Russ. J. Appl. Chem. 1998 71:584-588; Fedorov, N. F. and Samonin, V. V. Russ. J. Appl. Chem. 1998 71:795-798; Fedorov, N. F. Russian Chemical Journal 1995 39:73-83; Gordeev, S. K. and Vartanova, A. V. Zh. Prikl. Khimii 1994 67:1375-1377). TiC, B4C, Al4C3, TaC, and other carbides can also be transformed to carbon (Jacobson et al. J. Mater. Chem. 1995 5:595-601).
Closed-shell carbon structures such as multiwall nanotubes and carbon onions have been identified on the surface of CDC coatings produced by chlorination of SiC at about 1000° C. (Gogotsi et al. Nanostructured Carbon Coatings on Silicon Carbide: Experimental and Theoretical Study. in Proc. NATO ASI on Functional Gradient Materials and Surface Layers Prepared by Fine Particles Technology (eds. Baraton, M. I. & Uvarova, I. V.) (Kluwer, Dordrecht, 2000); Zheng et al. J. Mater. Chem. 2000 10:1039-1041; Jacob et al. In Amorphous and Nanoporous Carbon. Mat. Res. Soc. Symp. Proc. edited by Sullivan et al. Materials Research Society, Warrendale, Pa. (2000), Vol. 593, p 87). Nanotube-like and onion-like materials consisting of close-shell structures have been commercialized by Skeleton Technologies, Sweden, under the trade name Skeleton-C®. Production of these materials is performed at or below 1000° C. and no tube alignment occurs.
Aligned MWNTs have been produced at Japan Fine Ceramics Center by the thermal decomposition of SiC crystals in a vacuum (Kusunoki et al. Appl. Phys. Lett. 1997 71:2620-2622; Kusunoki et al. Phil. Mag. Lett. 1999 79:153-161). Similar experiments conducted at higher temperatures ranging from 1600 to 1750° C. on different SiC substrates produced very thin (2-5 nm in diameter) nanotubes including double-wall nanotubes and SWNTs, which had hemispherical fullerene-like caps (Kusunoki et al. Jpn. J. Appl. Phys. 1998 37:L605-L606). In these experiments, nanotube coatings on single crystals and powders, as well as free-standing nanotube films were produced. Further, reproducible synthesis of nanotubes using SiC28,29 or TiC30 as a precursor and carbon source was shown.
WO 01/16023 discloses a method for production of nanoporous nanotube-like carbon in large quantities involving halogenation of aluminum carbide at a temperature above 450° C. and preferably from about 500 to 850° C.